A computer-based recommendation service algorithmically selects items, e.g., goods (physical and/or digital) and/or services, to recommend to users. One common application for recommendation services involves recommending items for purchase, rental, subscription, viewing, or some other form of consumption. For example, some e-commerce sites provide services for recommending item to users based wholly or partly on the ratings assigned by the users to particular items. The recommendations may additionally or alternatively be based on the users' order histories, purchase histories, rental histories, item viewing histories, item tagging activities, and/or other behavioral profiles. Recommendation services are also commonly used to recommend web sites, news articles, music and video files, television shows, restaurants, and other types of items.
Some recommendation services present users with lists of items that are related to the item they are currently viewing. These lists may be based on aggregated purchase histories or item viewing histories of users, and are typically presented on catalog pages of an electronic catalog. For example, when a user accesses the item detail page for a particular item, the user may be presented with a list of items frequently purchased by those who purchase this item, and/or with a list of items frequently viewed by those who view this item. These lists assist users in identifying additional items that are complementary to, or are substitutes for, the item being viewed.